ADP2387ACPZN-R7 datasheet(16/25 Pages) AD

16 / 25 page

ADP2387

Data Sheet

Rev. C | Page 16 of 25

INDUCTOR SELECTION

The operating frequency, input voltage, output voltage, and

inductor ripple current determine the inductor value. Using a

small inductor leads to a faster transient response, but it degrades

efficiency due to a larger inductor ripple current; whereas using

a large inductor value leads to smaller ripple current and better

efficiency but results in a slower transient response.

As a guideline, the inductor ripple current, ΔIL, is typically set to

one-third of the maximum load current. Calculate the inductor

value by using the following equation:

L =

OUT

∆

−

)

(

where:

VIN is the input voltage.

VOUT is the output voltage.

D is the duty cycle (D = VOUT/VIN).

ΔIL is the inductor current ripple.

fSW is the switching frequency.

The ADP2387 uses adaptive slope compensation in the current

loop to prevent subharmonic oscillations when the duty cycle is

larger than 50%. The internal slope compensation limits the

minimum inductor value.

For a duty cycle that is larger than 50%, determine the

minimum inductor value by using the following equation:

L (Minimum) =

(

)

OUT

−

Calculate the peak inductor current as follows:

IPEAK = IOUT + 2

Δ L

The saturation current of the inductor must be larger than the peak

inductor current. For ferrite core inductors with a quick saturation

characteristic, the saturation current rating of the inductor must

be higher than the current-limit threshold of the switch. This

higher rating prevents the inductor from reaching saturation.

Calculate the rms current of the inductor as follows:

IRMS =

OUT

∆

Shielded ferrite core materials are recommended for low core

loss and low EMI. Table 7 lists some recommended inductors.

OUTPUT CAPACITOR SELECTION

The output capacitor selection affects the output ripple voltage

load step transient and the loop stability of the regulator.

For example, during a load step transient where the load is

suddenly increased, the output capacitor supplies the load until

the control loop can ramp up the inductor current. The delay

caused by the control loop causes output undershoot. Calculate

the output capacitance that is required to satisfy the voltage

droop requirement by using the following equation:

COUT_UV =

OUT

STEP

)

(

∆

−

∆

where:

KUV is a factor, with a typical setting of KUV = 2.

ΔISTEP is the load step.

ΔVOUT_UV is the allowable undershoot on the output voltage.

Another example occurs when a load is suddenly removed from

the output, and the energy stored in the inductor rushes into

the output capacitor, causing the output to overshoot.

Calculate the output capacitance required to meet the overshoot

requirement by using the following equation:

COUT_OV =

)

(

OUT

STEP

−

∆

where:

KOV is a factor, with a typical setting of KOV = 2.

ΔVOUT_OV is the allowable overshoot on the output voltage.

The equivalent series resistance (ESR) and capacitance value

determine the output ripple. Use the following equations to

select a capacitor to meet the output ripple requirements:

COUT_RIPPLE =

RIPPLE

OUT

∆

RESR =

RIPPLE

OUT

∆

where:

ΔVOUT_RIPPLE is the allowable output ripple voltage.

RESR is the ESR of the output capacitor in ohms (Ω).

Select the largest output capacitance given by COUT_UV, COUT_OV,

and COUT_RIPPLE to meet both load transient and output ripple

performance.

The selected output capacitor voltage rating must be greater

than the output voltage. The rms current rating of the output

capacitor must be larger than the value calculated by

OUT_RMS =

∆

ADP2387ACPZN-R7 数据表(PDF) 16 Page - Analog Devices

ADP2387ACPZN-R7 数据表(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表下载

链接网址

ADP2387ACPZN-R7 数据表(PDF) 16 Page - Analog Devices

ADP2387ACPZN-R7 数据表(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

数据表 下载

链接网址

数据表下载